Composite wood products are manufactured by binding together wood elements (e.g., veneers, strands, flakes, chips, particles, fiber, solid pieces of wood, etc.) with suitable adhesives. Such products are typically used in construction applications as beams, headers, columns, joists, rafters, studs, or other structural components. To be suitable for this type of use, composite wood products should exhibit a certain degree of stiffness and dimensional stability.
Parallel strand lumber (PSL) is a high-strength composite wood product marketed under the name Parallam™ from iLevel by Weyerhaeuser Company. FIG. 1 is a schematic showing a conventional method for manufacturing PSL. Typically, wood elements (indicated by arrow 102) and an adhesive (indicated by arrow 104) are combined in an adhesive application station 106. The coated wood elements then enter a forming station 108 where they are laid up in a substantially parallel relationship to form a mat. The mat is consolidated in a press assembly 110 to form a billet 200 (shown in FIG. 2) having four faces: a top face 202, a bottom face 204, and two side faces 206 and 208. The thickness T of the billet 200 is substantially greater than its width W. Referring back to FIG. 1, the resulting billet is typically cut into two or three beams with a cutting assembly 112. FIG. 3 illustrates how the billet 200 may be cut into three beams along dotted lines 302 and 304 according to conventional methods.
In some applications, PSL produced according to methods described in FIG. 1 exhibit bulging on the top face 202 and the bottom face 204. Because the top and bottom faces of the billet are typically the coolest after exiting the press, they are more prone to blows and fissures. As a remedy, some PSL manufacturing companies have added a top/bottom preheat step to the PSL manufacturing process. As illustrated in FIG. 4, the mat is exposed to a top/bottom preheat assembly 402 prior to entering the press assembly. FIG. 5 shows an example of a top/bottom preheat assembly 402 for use with the method described in FIG. 4. A mat 502 is shown passing through a preheat assembly 402 and a press assembly 110. The preheat assembly 402 includes an upper wave guide 504 and a lower wave guide 506. Microwave energy represented by arrows 508 and 510 is delivered to the top and bottom of the mat before pressing.
In all conventional PSL manufacturing methods, the process of cutting the billet into beams releases stresses induced by the manufacturing process. As shown in FIG. 6, this can result in a beam 600 that is not completely straight. Beams manufactured according to conventional methods tend to exhibit bowing (indicated by reference character 602) and often revert to a shape similar to but less severe than the shape of the platen radius in the press. This shape is undesirable and can result in the product being unsuitable for use in a particular application due to its dimensional instability. Although the systems and methods shown in FIGS. 4 and 5 may be beneficial for reducing bulging, they have also been shown to exacerbate the problem associated with bowing in PSL beams. Bowing effects are well documented in PSL manufacturing and similar problems occur during the manufacturing other types of composite wood products.
Wood product manufacturing companies have taken numerous steps to reduce bowing and other defects associated with composite wood product manufacturing. One method involves modification of the shape of platen used in pressing operations. If the press infeed is reshaped to have a substantially larger radius, bowing effects can be mitigated. One drawback of this approach is that retrofitting presses can be extremely expensive. In addition, retrofitting is not possible in some facilities due to space constraints. Another method for mitigating bowing in PSL involves reducing the line speed for PSL manufacturing processes. Although this approach has been shown to reduce bowing to some extent, imposing line speed restrictions can ultimately lead to significant financial losses due to reductions in the amount of product that can be produced.
Accordingly, there is a need to develop new systems and methods for manufacturing PSL and other composite wood products to reduce and/or eliminate bowing and other undesirable effects from manufacturing stresses. Ideally, a process capable of reducing bow in PSL beams (when compared to conventional methods) without reducing throughput would be extremely useful in the composite wood products manufacturing industry.